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 To replace substrings in a file that are at the beginning of a line,
 do this:
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 sed -i 's/^a\tb.*$/<replacement text>/g' <file>
 Where:
 ^ - Represents beginning of file/input,
 \t - Represents tab symbol,
 . - Any character,
 * - Zero or more of the preceding expression
 $ - End of input
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 In a real life case, it was used like this:
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 sed 's/.*<expression to look for>.*/<substitution pattern>/g' file_name
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 https://tinyurl.com/ygfk75nc